I am having a problem with my Halstead Finest Boiler. The Heating works ok by itself, but when a hot water tap is turned on and the heating is off, it sometimes fires up but most times the boiler goes to lockout with a continuous red light. Once the electric is switched off and on again it goes to standby mode ok but once the tap is turned on again it goes back to lockout. If the boiler is on for heating and a tap is turned on, it sometimes works, sometimes the boiler goes to lockout shutting off the heating also. Also when trying to run a bath, the boiler keeps shutting down and firing up again and eventually going to lockout. It seems the problem is with the demand for hot water. Could it be the flow switch or the thermistor not working correctly or could there be a problem with the pcb. The Heating side appears to be working ok by itself. Any ideas???